1. Field of the Invention
The present invention relates to antenna devices and communication terminal apparatuses and, in particular, to an antenna device and communication terminal apparatus preferably for use in a communication system in the high-frequency (HF) range.
2. Description of the Related Art
A radio-frequency identification (RFID) system for exchanging information between a reader-writer and an RFID tag by non-contact communications between the reader-writer and the RFID tag and a near field communication (NFC) system for carrying out communications between two communication apparatuses at a short range are known. An RFID system and a near field communication system that use the HF range, for example, a 13.56 MHz range, as the communication frequency employ antennas coupled to each other mainly through an induction field.
In recent years, some communication terminal apparatuses, such as cellular phones, have introduced an RFID system or a near field communication system, and the communication terminal apparatuses have been used as an RFID tag and a reader-writer used therefor or been used as terminals in near field communication. A magnetic antenna is known as an antenna device for use in transmitting and receiving a radio-frequency signal in the HF range. The magnetic antenna has a structure in which a coil conductor is wound around the surface of a magnetic core, as described in, for example, Japanese Unexamined Patent Application Publication No. 2005-317674 and Japanese Unexamined Patent Application Publication No. 2007-019891.
FIG. 1 is an exploded perspective view of a magnetic antenna in Japanese Unexamined Patent Application Publication No. 2007-019891. The magnetic antenna is a laminate that includes a plurality of magnetic layers 5 in which a coil 4 made up of electrode layers 2 and through holes 1 is disposed, insulating layers 6 sandwiching the upper and lower surfaces of the coil 4, and a conductive layer 7 disposed on the upper surface of one or more of the insulating layers.
In a system that uses the HF range as the communication frequency, the communication distance between the antenna devices depends on magnetic flux passing through the coil antennas. That is, to ensure some communication distance between the antenna devices, it is necessary for each of the coil antennas to have a large size. The large size of the coil antenna hinders miniaturization of the communication terminal apparatus. In contrast, if the antenna is small, the effective area of the antenna is small and the communication distance is not sufficient.